A common problem in animation is creating animated characters that move along a specified path. There are several ways to specify this kind of motion. One way is to use motion capture data. Another way is to specify a series of key frames to describe the motion using a combination of inverse and forward kinematics and character rigging techniques.
In both motion capture and dense key frame animation, it can be difficult to visually edit motion that shifts between being locked at a single point in space and being in motion. The most common example of this scenario is way a foot locks to a position on the ground, accelerates to step forward and then decelerates to a new locked position. This problem is particularly acute when the animation is defined using motion capture data. For example, an animator may have a motion capture data representing a person walking. Repositioning where the character is stepping is currently a great deal of work, and typically requires laborious manual editing or offsetting of motion curves.
FIG. 1 illustrates the typical problem. The motion capture data represents a character walking, shown over time with the line 100. The animator desires to reposition the character""s feet over time, for example to the line 102 by manipulating the motion capture data. This task of editing the motion capture data for repositioning the character typically requires a lot of work.
Current solutions to this problem include describing original motion capture rotation information in reference to a path and blending in foot stepping positions only at points where the foot is in contact with the ground. The foot contact positions are introduced as a secondary layer. Such a solution is described in xe2x80x9cMotion Editing with Spacetime Constraints,xe2x80x9d by Michael Gleicher, in Proceedings of the 1997 Symposium on Interactive 3D Graphics and related work.
The problem of editing motion data can be solved by providing a way to specify control points (herein called xe2x80x9chandlesxe2x80x9d) along the path of the motion data and to describe the motion data as a combination of layers of information in relationship to these handles.
For example, a first layer may describe, for each point in the motion data, the distance of the point between the handles. For example, a path between two handles may be defined. Each point in the motion data is closest to a point along that path, which may be called a reference point. That point along the line has a distance to the two handles. These distances may be defined as a percentage of the length of the path. A second layer may describe the offset of points in the motion data from the line between the two handles.
In one embodiment, the handles may be identified by the animator either by spatial position, timing or a combination of the two. In another embodiment, the handles may be identified automatically, for example, at predetermined intervals in the motion data or by identifying points where motion stops, changes course, or dips below a minimum velocity.
To edit an animation using the motion data, an animator simply can manipulate the handles in three-dimensional space and/or manipulate the offsets.
Thus, by describing motion as a series of connecting handles, and a combination of layers describing distances and offsets along a path between the handles, the motion data can be easily modified using a graphical user interface that allows an animator to reposition the handles and modify the offsets.